Method and material for applying phosphate conversion coatings on zinciferous surfaces



United States Patent 3,109,757 METHOD AND MATERIAL FOR APPLYING PHOS-PHATE CGNVERSION COATINGS ON ZINCIF- EROUS SURFACES Earl R. Reinhold,Levittown, Pm, assignor to Arneheln Produc w, Inc., Ambler, Pa, acorporation of Delaware N0 Drawings. Filed Jan. 26, 1962, Ser. No.169,997 6 Citations. (Cl. 148--6.15)

This invention relates to the art of coating zinciferous surfaces andinvolves a method and solutions for producing phosphate conversioncoatings on such surfaces which coatings have highly improvedcharacteristics, especially with respect to fineness of grain structurewithout serious sacrifice of coating weight, ability to promote theadhesion of siccative finishes and resistance to stresses set up bybending, folding or shaping the metal after the coating has beenapplied.

As used herein the expression zinciferous surfaces or zinc surfaces areto be understood as applying to alloys of zinc with other metals whereinzinc is the principal ingredient, to hot-dipped galvanized surfaces, toelectrodeposited zinc surfaces, as well as to essentially pure zincsurfaces.

The art of applying phosphate conversion coatings to zinc surfaces, ofcourse, is well developed and is based upon the familiar practice ofsubjecting the surfaces to the action of an aqueous acid phosphatecoating solution until the desired coating has been deposited. Followingthis the surface is customarily rinsed either with water or with dilutechromic and/or phosphoric acid rinses. Probably the most familiar of theaqueous acid phosphate coating solutions is a zinc solution and thepresent invention is directed to the improvement of methods involvingsuch a solution.

In this art of coating zinc surfaces with aqueous acid zinc phosphatecoating solutions various solution formulations are well known to theart and the present invention is broadly applicable to solutions of thisgeneral type although, as will be pointed out below, maximum benefitsfrom the present invention are obtainable where the aqueous acid zincphosphate solutions conform to the specifications recited hereinafter.

In the phosphate coating of zinc surfaces the art has long realized thatthe conversion coated surfaces and any siccative finish that may havebeen applied thereto do not lend themselves very well to bending orfolding without serious attendant cracking and peeling with subsequentloss of or injury to the siccative film. This is particularlyexemplified by what happens to an acrylic base paint which is oftenobjectionably impaired as the result of cracking of the phosphatecoating in situations requiring bending or shaping operations such, forexample, as in the use of galvanized iron sheeting as siding in theconstruction industry.

With the foregoing in mind, the principal object of the presentinvention is the provision of a method and solutions for applying animproved, adhesion-promoting, zinc phosphate conversion coating on zincsurfaces which will withstand or resist to a marked extent flexing,shaping and bending operations even to an extent involving a 180 bendwithout failure of the siccative finish or film.

The invention is broadly based upon the discovery that if the followingagents are added to an aqueous acid zinc phosphate coating solution ofgenerally standard characteristics and such solution is then utilized inthe treatment of Zinciferous surfaces it will formon the surfaces zincphosphate conversion coatings which are smoother, finer grained, moretightly adherent, unimpaired as to coating weight, and better suited toserve as a substrate for subsequent sicca-tive finishes than are thefamiliar zinc ice phosphate conversion coatings presently known to theart. The addition agents employed with my invention are:

(a) From 0.1% to 1.0% by weight of a compound selected from the groupconsisting of glycerophosphoric acids and water-soluble salts thereof;and

(b) At least 0.025% by weight of complex fluoride radical selected fromthe group consisting of fluoboric, fluosilicic, fluotitanic andfluozirconic acids and alkali metal and ammonium salts thereof.

The glycerophosphoric acids or water-soluble salts there- 'of includemono and di-glycerophosphate compounds. Soluble salts are preferred overthe acids in view of their greater solubility in the acid phosphatecoating media. Particularly preferred salts include the alkali metal andammonium mono and di-glycerophosphates, since these are more readilyprepared, and provide an easily handled concentrated liquid solution.

Where the amount of glycerophosphate, or water-soluble salt thereof,which is employed, is less than the 0.1% by weight, minimumconcentration, the improved paintadhesion promoting coatings will not beobtained, and subsequent bending or flexing of the phosphate coated andpainted zinc surfaces will result in cracking and peeling of thesiccative film. As the amount of glycerophosphate compound in thephosphate coating solution is increased, the paint bonding qualities ofsubsequently produced coatings on zinc surfaces also increases up to anapparent maximum improvement at about 1% concentration, by weigh-t. Useof glycerophosphates in amounts greater than 1% concentration has nonoticeable deleterious effect upon either the coating reaction, or uponsubsequent paint adhesion on the coated surfaces. However, as noadditional benefit is obtained when employing more than about 1%glycerophosphate, it is preferred, in the interests of economy and toprevent waste, to operate between a range of from 0.1% to 1.0%concentration thereof as stated above.

As for the quantity of complex fluorides, I have found that, in order torealize the benefits of this invention, they should be added in anamount equal to at least 0.25% by weight of the coating solution. Ifless than this minimum quantity is present the resulting conversioncoatings will not provide the highly flexible, paint-bonding qualitiesobtainable by my invention.

Insofar as an upper concentration limit of complex fluoride isconcerned, I have found that there is nothing critical about thequantity employed since solubility considerations pretty well limit theamounts thereof which can be dissolved in zinc phosphate coatingsolutions of this general character. For example, the complex fluorideacids, i.e. fiuoroboric, fluosilicic, etc., and the ammonium saltsthereof have been found to be more soluble in the coating baths orsolutions than the alkali metal salts. However, the maximum solubilityof any of these complex fluoride compounds in the customary aqueous acidzinc phosphate coating solutions is about 2.5% by weight thereof. Use ofa relatively low concentration of complex fluoride compound, forinstance, about 0.03%, provides the same enhanced adhesion-promotingcharacteristics as does the use of large amounts, even those amountswhich approach the saturation limitation. Therefore, in the interests ofpreventing waste and securing maximum economy of operation, I prefer toemploy a range of complex fluoride concentration of from 0.005% to 0.25%by weight of the coating solution.

While the use of complex fluorides from the class described is essentialfor successful operation of the process of this invention, it has beenfound that the inclusion of simple fluoride ions into the coatingsolution of this invention has no apparent deleterious effect uponcoating formation. However, use of simple fluoride ion alone will notprovide the improved results obtainable with the complex fluorides asdefined hereinabove.

As a further guide in attainment of the maximum benefits of my inventionI wish to call attention to the followmetals are employed) of less than0.3% of nickel and as little as 0.0003% to 0.001% of copper.

A concentrate which is suitable for dilution with water, as hereinafterdescribed, to produce the coating solutions i .f of this invention, andwhich is also suitable for addition The phosphate ion content whichyields the best results to partially exhaust solutions as atreplenisher, can be ranges from about 0.5% to about 2.5% by Weight ofthe prepared according to the following exampl coating solution with theminimum quantity being at least sufiicient to form the dihydrogenphosphate with the zinc EXAMPLE I ions present and the content of zincions should be in Percent by Welght an amount which does not exceedabout 0.5% by weight Zno of the coating solution. H?PO4 (75%) Anotherfactor which should be observed in obtaining 3 9 the maximum benefits ofmy invention arises in connec- 3 (38 tion with the presence of anyoxidizing ion such as nitrate Water 42-88 and mixtures of nitrate andnitrite. These oxidizing agents, of course, are very familiar to thoseskilled in the 100-00 art and for best results with my invention thequantity h abovg f l h dil d i h watmto a of nitrate ion should be about0.2% to about 1.0% with Strength f 4% by volume d h i dd d h m majorbenefits being Obtalnable when the l f 'fiy 116$ 0.25% (weight/volume)of caustic soda, has the followbetween about 0.3% to about 0.5%. Wherenitrite 1s also ing analysis; included in the bath as an oxidizing agentsubstantially T 1 smaller amounts thereof are required in order to yieldQ i i 3 0 the results which are secured by this familiar ingredient. i-i0 For example, the range of nitrite ion when used in com- 25 erwm mobination with nitrate ion is from about 0.0002% to about The acidityreferred to was determined by the method 0.008% and with my invention Ihave found that optimum indicated above. flexible paint-bondingproperties in the phosphate con- To a dilute solution prepared as abovedescribed there version coatings are obtainable with the use of lessthan should then be added from 0.1% to 1.0% by weight of about 0.5%nitrate ion and less than about 0.005% of a glycerophosphate compound ashereinabove described nitrite ion. and at least 0.025% by weight of acomplex fluoride as Insofar as total acidity of the solution isconcerned, previously described. The solution should then be heated Ihave found that it is preferable to utilize zinc phosphate to betweenabout 120 F. and 160 F. and preferably coating solutions which have atotal acidity of less than between 130 F. and 160 F. and can then beutilized with 40 points. Total acidity is the milliliters of tenthnormal greatest benefit in applying zinc phosphate conversion sodiumhydroxide (points) required to titrate a 10 ml. coatings to zinciferoussurfaces and it will be found that sample of the bath to aphenolphthalein endpoint, whereas it will produce excellent, highlyflexible, smooth and finethe free acidity is the mls. of tenth normalsodium hydroxgrained, paint-bonding phosphate conversion coatings ide(points) required to neutralize a 10 ml. sample of which are ideallysuitable as a substrate for siccative the bath to a brom cresol greenendpoint. finishes.

Beyond the foregoing, it an aqueous acid zinc phos- The advantagesattainable with my invention are amply phate coating solution isemployed which contains nickel demonstrated by the following comparativetests which I and/or copper ion (these ions of course being well haveconducted. A 4% dilution of the concentrate of known to the art), it isimportant to observe the following Example I given above was preparedand aliquot samples facts in order to secure the maximum benefits of myinof the diluted solution were utilized in accordance with vention. Inthe case of nickel ion there should be emthe examples given in Table Ibelow. The metal being ployed somewhere between about 0.01% and 0.4% andin coated was hot-dipped galvanized steel. A series of simithe case ofcopper between about 0.0003% and 0.005%. lar panels from such galvanizedsteel were prepared and Use of higher amounts than these of nickel andcopper cleaned in accordance with well established prior art pracwillnot yield the full advantages otherwise obtainable tices. The reportedpaint adhesion results were deterwith the process and solutions of myinvention. Actually, mined following application of an epoxy primarypaint best results are secured with the utilization (where these system.

Table I Complex fluoride Paint adhesion test (ASTM D52241) ExamplePercent Coating N o. glycerowt.,

phosphate rug/ft. Percent Percent Type paint Paint appearance loss 2None None 462 50 Heavy cracking.

3 None 0.05 (NH4)2ZHFB 487 10 Moderate crack- 4 None 0.126 HzSiFs 469 30H e y cracking.

5 None 0.033 HZTiFB- 442 20 D0.

6 0.1 None 501 5 Fine cracking.

7 0.1 0.05 (NHQzZnF n 504 1 Very fine cracking. s 0.2 0.05 N11 2111 4201 Do.

9 0.4 0.05 (N114): Z1111. 402 1 Do.

10 0.1 0.126 H2SlFfl 477 2 Fine cracking.

11 0.2 456 1 Very fine cracking. 12 0.4 468 1 Do.

13 0.2 417 1 l4 0. 2 494 1 Very fine cracking.

1 Examples 8 and 14 also contained 0.005% copper ion.

2 The panels of Example 13 were inadvertently lost and were notavailable for this test but visual observation thereof indicated thattheir coatings would be equally satisfactory as those in Examples 7 to16, inclusive.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,109,757 November 5 1963 Earl R. Reinhold It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 2, line 40, for "0.25%" read 0.025% column 4, line 6, for"exhaust" read exhausted Signed and sealed this 28th day of April 1964.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J, BRENNER Commissioner ofPatents

1. IN THE ART OF PHOSPHATGE COATING ZINCIFEROUS SURFACES,M THE METHODWHICH COMPRISES SUBJECTING SAID SURFACE TO THE ACTION OF AN AQUEOUS ACIDZINC PHOSPHATE COATING SOLUTION, THE COATING PRODUCING INGREDIENTS OFWHICH CONSIST ESSENTIALLY OF PHOSPHATE ION FROM 0.5% TO 2.5% BY WEIGHTAND NITRATE ION FROM 0.2% TO 1.0% BYW EIGHT, WHICH SOLUTION ALSOCONTAINS AS ADDITION AGENTS: (A) AT LEAST 0.1% BY WEIGHT OF THE SOLUTIONOF A COMPOUND SELECTED FROM THE GROUP WHICH CONSISTS OFGLYCEROPHOISPHORIC ACIDS AND WATER SOLUBLE SALTS THEREOF; AND (B) ATLEAST 0.025% BY WEIGHT OF THE SOLUTION OF COMPLEX FLUORIDE RADICALSELECTED FROM THE GROUP CONSISTING OF FLUORBORIC, FLUOSILICIC,FLUOTITANIC AND FLUOZIRCONIC ACIDS AND ALKALI METAL AND AMMONIUM SALTSTHEREOF; AND REPENLISHING THE SOLUTION AS REQUIRED TO MAINTAIN ITSINGREDIENTS AS DEFINED.